The present invention relates to the art of electric arc welding by a robot driven welding gun and more particularly to a novel probe used for touch sensing the position of the workpiece in a robot welding application.
In welding where an electric arc welding gun is moved along a preselected path by a robotic mechanism, it is first necessary to determine the position of the workpiece prior to initiation of the welding operation. This can be done manually; however, mass production and operator free robot welding installations require a system for determining the location of the workpiece and, thus, orienting the gun with respect to the welding process to be accomplished. In practice this orientation procedure involves a computer program wherein the welding wire extending from the gun is moved into engagement with the workpiece at several positions to determine the actual spatial location of the welding path. The use of the welding wire to repeatedly engage the workpiece for sensing the position of the workpiece creates substantial difficulty, especially when the welding wire is aluminum. The torch or gun is moved by the robotic arm in several directions to use the welding wire as a touch sensing feeler. The several points detected by the wire generate the orientation of the workpiece. The wire must touch the workpiece several times and it must not bend, deflect or be forced back into the welding gun. It is often necessary to include a brake in the gun for holding the welding wire in the gun so it remains fixed with respect to the gun during the touch sensing sequence controlled by the computer operating the robot. The wire must extend from the gun a known fixed distance. This requires clipping of the wire and then extending the wire from the gun before the touch sensing sequence can be performed by the computer generated sequence of movements for the sensing procedure. To assure that the welding wire is not pushed back into the gun, the robotic touch sensing sequence often involves moving the gun at an angle with the wire extending out of the gun at an angle. This produces deflection of the wire and some inconsistencies in the location of the weld to be performed. Since the wire is quite fragile, the gun must be moved slowly during each of the touch sensing steps used to generate points for creating the location of the workpiece. This is the general background to which the present invention is directed.
In Debarbieri U.S. Pat. No. 4,642,752, a rigid feeler is mounted adjacent the welding torch or gun in a manner to detect a fixed portion of the workpiece to register the location of the workpiece. This type of fixed feeler has not been used extensively since it inhibits operation of the welding gun in confined areas. However, this patent is incorporated by reference herein as background information. Michelotti U.S. Pat. No. 4,530,456 and Mawson U.S. Pat. No. 4,284,871 show the use of a vertical sensing probe used to locate the height of a torch above the workpiece. Such probes operate in the Z axis and merely move downwardly with the soldering or welding torch until the probe is against the workpiece. This locates the height of the torch. There is no use of these devices to locate the path to be welded during a robotic welding operation. These two patents are also incorporated by reference as background information.
Toohey U.S. Pat. No. 4,439,664 is an example of a welding torch that uses the wire extending from the torch as a probe to locate the seam being welded. This disclosure is similar to the general concept used for robotic arc welding processes in that the welding wire itself forms the outwardly depending locating feeler. This particular patent shows a vertical movement of the welding gun for the purposes of locating the workpiece and not an angled use of the welding wire as is normal practice in arc welding processes now being performed in computer controlled robotic equipment. Toohey is also incorporated by reference as background information.
The present invention relates to a probe for touch sensing to determine the position of a workpiece in a robotic welding system and an improved system using the novel probe. In accordance with the invention, a rigid, elongated feeler with a touch tip is mounted in a mechanism adjacent the welding gun of the robotic welding system so that the contact point of the touch tip can be extended by instructions from the computer program to a fixed distance beyond the end of the welding gun. In this fixed extended position, the feeler is moved by a computer program to touch the workpiece at many locations. In practice, six to eight touches are used to determine the exact position of the workpiece preparatory to the welding operation. The feeler is fixed with respect to the welding gun so that the determined orientation calculated by the computer will determine the exact spatial location of the welding path to be processed by the computer movement of the welding gun. The invention does not present the problems associated with using the extended welding wire as the feeler moved into several locations for the purposes of generating a computer analysis of the exact location of the workpiece. When using the wire for the purposes of touch sensing, the wire can only be moved downward by the movement of the welding gun at a rate of about 15 millimeters per second. By using the present invention, the probe can be moved by the robotic arm at a rate of approximately 200 millimeters per second. Consequently, the six to eight steps used in a touch sensing program are performed using the present invention in about eight minutes. The same process using the extended welding wire requires over twenty minutes and causes bending of the wire causing less accurate location. Indeed, the wire must be held in the gun, clipped and extended a fixed distance before the wire is useable for the touch sensing program. The present invention allows the fixed feeler to be used rapidly without concern for distortion during the processing. When seven touches are necessary for locating a weld position, the use of the fixed feeler of the present invention is extremely advantageous by saving time and resulting in better accuracy.
The invention involves the novel probe and also the concept of retracting the probe after the touch sequence has been processed by the computer program used for the robotic welding of the workpiece. The probe is parallel to the wire and is moved into the extended operative position. Thereafter, it is retracted into an inoperative position where it is not capable of interference with movement of the welding gun during welding. The wire extending from the welding gun is not used for the location of the workpiece. Thus all of the disadvantages previously associated with such procedure are avoided. Although it is preferable that the feeler or probe be parallel to the welding gun the probe may be at a fixed angle to the welding gun. This position of the probe or feeler introduces complexity in the computer programming and also cause a certain amount of inaccuracy in the touch sensing process. By using the invention, there is no need to provide a brake in the gun to hold the welding wire in a fixed position for touch sensing. Furthermore, it is not necessary to contact the workpiece at an angle although this can be done if the computer program requires such orientation of the probe constructed in accordance with the present invention.
The retractable touch sensing probe is used on an arc welding robot to aid in the sensing of workpiece orientation. It is rigid and retractable so that there is no deformation of the sensing device. The cycle time is substantially reduced and the probe retracts, allowing unrestricted weld torch or gun movement along the weld path. In the practical implementation of the present invention, the probe consists of a pneumatic cylinder using a two way pneumatic solenoid valve to shift the rigid feeler into the extended touch position or the retracted operative position. A standard output bus merely activates the solenoid valve to move the probe outward into the operative position. The electromechanical device for operating the solenoid valve, in one embodiment also functions to close a switch that applies a voltage to the probe. This voltage is then measured by the computer or electric arc welder for indication of a touch to the workpiece by a detector. When the voltage level plunges, contact of the feeler or probe with the workpiece is detected. By using standard robot touch sensing software, the novel probe is linked to the touch sense I/O outlet to determine when the probe is extended for the processing inputting data calculated by the software program. The same digital output that extends the probe also connects the probe to a positive output voltage. This creates an open circuit with the touch sensing voltage being the voltage between the probe and the workpiece. When the robot touches the workpiece, the voltage potential between the two points drops. This is detected by either the robot controller or the welding power supply if equipped with a standard touch sensing capability. After the various touches have been recorded to calculate the position of the workpiece, the probe retracts so it no longer extends past the torch body of the gun, thereby allowing the welding process to proceed uninhibited.
In accordance with the present invention, there is provided a probe for touch sensing to determine the position of a workpiece in a robot welding system using a robot driven welding gun for advancing a welding wire in a given direction toward the workpiece. The welding gun or torch body is movable by a robot controlled arm in a selected weld path. The probe comprises a rigid, elongated feeler with a touch tip. The feeler is carried by a mechanism fixed with respect to the gun or torch body and movable by the mechanism between a retracted position and a fixed, known extended position wherein the feeler extends from the mechanism a fixed distance having an outermost contact point for the touch tip. This point has a known relationship to the gun. A detector detects when the contact point touches the workpiece. This is done by applying a voltage to the feeler to present a known voltage between the feeler and workpiece. When the feeler touches, the voltage decreases and is detected as a xe2x80x9ctouchxe2x80x9d in the touch sensing program sequence of the robot.
In accordance with another aspect of the present invention, the feeler extends parallel to the extended direction of a gun or torch body. Thus, when the feeler is extended, it is moved by the robot during the touching sequence just as the torch would be moved, but with a certain offset. In practice, the moving mechanism is a cylinder mounted on the gun and the feeler is carried by a piston movable in the cylinder when the feeler moves between the two positions. A lower chamber in the cylinder and below the piston receives pressurized fluid from a electromechanical operated valve to introduce the fluid into the lower chamber. This moves the feeler from the extended position to the retracted position. It is possible to force the probe in the extended position by use of a spring with the fluid pressure moving the feeler against the spring to retract the feeler. In a like manner, a chamber above the piston is used in practice to shift the feeler into the extended position. When this occurs, the feeler moves against the stop in the cylinder to set the outermost contact point of the feeler. The valve for directing fluid either to the upper chamber or the lower chamber is operated by electromechanical device, such as a solenoid operated by a relay. In practice, one switch is closed by the electromechanical device to connect the voltage to the feeler. Another switch is closed to shift the valve between the retract position and the extended position. In practice, the switches are operated optically to prevent noise.
In accordance with another aspect of the invention there is provided a touch sensing system for a computer controller robot welding system wherein a robot driven welding gun advances a welding wire in a given direction toward the workpiece. The welding gun is movable by a robot controlled arm in a selected weld path controlled by the computer. The system comprises a computer program to output a touch sense signal and a probe including a rigid, elongated feeler with a touch tip and having an extended position and a retracted position. An electromagnetic device is used for closing a first switch and second switch upon creation of the touch sensing signal. A fluid cylinder shifts the feeler into the extended position upon closing of the first switch. A voltage source is connected to the feeler upon closing of the second switch. A detector is used for detecting a reduction in the voltage to indicate touching of the feeler against the workpiece. The program also creates several movement steps to sequentially move the feeler against the workpiece at diverse locations so a program processes the detections of the steps to orient the gun with respect to the workpiece. This system can be used on all robots using touch sensing to locate the welding path.
The primary object of the present invention is the provision of a probe to be used in a robot welding system. The probe includes an extended active position and a retracted inactive position.
Yet another object of the present invention is the provision of a probe, as defined above, which probe is rigid and elongated and overcomes the disadvantages of using a flexible welding wire as the feeler for a touch sensing sequence in a computer controlled robot system.
Still a further object of the present invention is the provision of a system for touch sensing to determine the position of a workpiece. The system involves the use of a novel probe, as defined above.